Hearing protector

ABSTRACT

An active in the ear hearing protector having an ear piece (13) and a casing (12) attachable to the ear piece, the ear piece being positionable in the ear canal of a user so as to peripherally acoustically seal the ear canal and such that the attached casing (12) is positioned externally of the ear canal. The casing (12) incorporates a microphone (14) for receiving external acoustic sound signal and a signal processing circuit (15) for receiving electrical signal from the microphone and correspondingly causing a loudspeaker (16) in the casing to generate sound signal for direction down a tube (34) extending through the ear piece to be transmitted to the ear canal. The electrical circuit (15) is arranged to limit the electrical signal delivered to drive the loudspeaker whereby to limit the sound pressure of sound signal delivered from the loudspeaker to the ear canal via the tube (34).

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to a hearing protector.

2. Description of the Prior Art

There are situations in military services, industry, sports involvingthe use of fire arms, aviation and the like, where persons are exposedto loud sounds at sound pressure levels in excess of those which areknown to cause permanent hearing damage. To minimize the degree ofheating damage incurred in these situations, hearing protectors in theform of earplugs, earmuffs, or a combination of earplugs and earmuffscan be worn. These hearing protectors can be classified as passive oractive, and linear or nonlinear.

LINEAR-PASSIVE HEARING PROTECTORS: Linear protectors of the passive typein the form of earplugs or earmuffs are designed to attenuate incomingsound waves of different intensifies by approximately the same number ofdecibels before they reach the eardrum--that is, the mount ofattenuation is independent of the sound pressure level of the incomingsound wave. The disadvantage of these linear hearing protectors of thepassive type is that, although the wearer is protected from loud sounds,his/her ability to hear speech sounds and warning signals at soundpressure levels which normally produce low to medium loudness in theabsence of the protector is impaired.

NONLINEAR-PASSIVE HEARING PROTECTOR: A nonlinear protector of thepassive type may be in the form of an earplug with a small aperture. Theattenuation characteristics of this type of earplugs are level-dependentand therefore nonlinear in that only incoming sound waves at levelsgreater than, say, 110 dB SPL are attenuated. These nonlinear earplugstherefore provide protection against intense impulse noises such asgunfire which produces peak sound pressure levels in excess of 110 dBSPL, but transmit sounds unattenuated at levels below 110 dB SPL. Onedisadvantage of these nonlinear earplugs of the passive type is that, atlevels greater than 110 dB SPL, they provide less attenuation than thelinear earplugs. Another disadvantage is that exposure to unattenuatedloud sounds at levels greater than 90 dB SPL is known to cause permanenthearing damage. For a discussion of the attenuation characteristics ofthese nonlinear-passive hearing protectors, see for example E. A. G.Shaw, "Hearing Protector Design Concepts and Performance Limitations",Personal Hearing Protection in Industry, P. W. Alberti Editor, 1982,Raven Press, N.Y.

NONLINEAR-ACTIVE MUFF: U.S. Pat. No. 4,064,362 (Williams); U.S. Pat. No.3,952,158 (Kyle et al.); and U.S. Pat. No. 3,394,226 (Andrews) describenon-linear protectors of the active type in the form of an muff, beingdesigned to improve communication. A protector of this type contains,inside an earmuff enclosure, a microphone, an electronic circuit, asmall loudspeaker and a battery. The microphone converts the incomingsound wave originating exteriorly of the earmuff enclosure to anelectrical signal which is processed by the electronic circuitcontaining a nonlinear compression amplifier and other signalconditioning circuits. For incoming sound waves at sound pressure levelswhich normally produce low to medium loudness in the absence of theprotector, the amplification characteristic of the compression amplifieris approximately linear with a fixed gain. For incoming sound waves atsound pressure levels which normally produce loud hearing sensations,the amplification characteristic of the compression amplifier isnonlinear in that its gain is inversely related to the sound pressurelevel of the incoming sound wave. The electrical output signal of theelectronic circuit is delivered to the loudspeaker which converts theelectrical signal back to sound inside the earmuff at the pinna. Theelectronic components of the electronic circuit arc chosen so that, forincoming sound waves which normally produce low to medium loudness, thesound pressure level at the pinna produced by the loudspeaker isapproximately the same as that produced by the incoming sound wave inthe absence of the earmuff. On the other hand, for incoming sound waveswhich normally produce loud hearing sensations, the sound pressure levelat the pinna produced by the loudspeaker is less than that produced bythe incoming sound wave in the absence of the earmuff as a result of thenonlinear behaviour of the compression amplifier. This protector offersunattenuated communication for speech and warning signals at low tomedium sound pressure levels against a quiet background, and providesprotection against harmfully loud noises by an mount approximately equalto that provided by a passive muff. The major disadvantages of thisactive protector include (i) difficulty in maintaining an adequatecontact between the head of the wearer and the seals of the earmuff dueto interference from ,the temple bars of safety glasses and/or longhair; (ii) poor condition of the seals; (iii) poor headband tension,(iv) bulkiness when used in conjunction with other safety devices suchas welding shields, hard hats, helmets, respirators, or a combination ofthese safety devices; (v) discomfort due to the weight of the earmuffand the irritation in an environment of high ambient temperature or highhumidity. Note that these disadvantages also apply to passive earmuffs.

NONLINEAR-ACTIVE CUSTOM MOULDED PLUG: Another nonlinear active protectoruses the same electronic principles as the nonlinear active muff, but isin the form of a custom moulded in-the-car hearing aid with miniaturehearing aid components housed in a custom-made earmould for fightfitting into the outer ear of the wearer. The miniature componentsinclude a microphone, a small loudspeaker (also known as a receiver), anelectronic circuit and a battery. As in the ease of the nonlinear activeearmuff, this in-the-ear protector offers unattenuated communication forspeech and warning signals at low to medium sound pressure levelsagainst a quiet background, and provides protection against harmfullyloud noises. The major disadvantages of this device include: (i)difficulty in achieving a fight fit between the earmould and outer earusing existing moulding techniques for adequate sound attenuation; (ii)difficulty in maintaining the original fit due to wear, tear andshrinkage of the custom-made earmould; (iii) increased costs because thecustom-made moulds must be fired by experienced technicians; (iv)clogging of the sound transmission path in the earmould by ear wax anddifficulty in removing the ear wax because only the ear canal end of thesound mission path is accessible to the user;, (v) increased maintenancecosts because the units must be sent back to the manufacturer or dealerfor proper maintenance to remove ear wax and to repair damage caused byusers who tried to clean out the wax; and (vi) lack of transferabilityfrom user to user because of the custom-made earmould. Thesedisadvantages are commonly encountered generally in the use ofin-the-ear hearing aids.

Other Related Art

U.S. Pat. No. 5,002,151 (Oliveira et al.) discloses a disposablecompressible polymeric foam sleeve to be disposably attached to ahearing aid. In the Oliveira et al. patent, the foam sleeve "can becompressed to be fully insertable into a person's ear and allowed torecover to become wedged in the ear canal", and it is preferablyattached to the hearing aid by mating of screw threads. However, thefoam sleeve in the Oliveira et al. patent is not intended for use inheating protectors and does not include adequate acoustic seals whichare required in hearing protectors to reduce substantially the directtransmission of incoming sound waves off,hating exteriorly of the outerear to the eardrum.

SUMMARY OF THE INVENTION

In accordance with the present invention, there is provided a hearingprotector comprising a sound processor for receiving at an input thereofexterior sound and generating therefrom at an output thereofcorresponding processed sound limited to intensifies not substantiallyexceeding a predetermined sound pressure level, and an ear piecedetachably mounted to said sound processor and having sound channelingmeans for passage of said processed sound away from said soundprocessor, said hearing protector being insertable into the outer ear soas to position said sound processor for receiving at said input saidexterior sound, being sound originating exteriorly of the outer ear, andfor directing said processed sound, as generated from said exteriorsound by said sound processor, to the eardrum via said sound channellingmeans of said ear piece, while substantially precluding directtransmission of said exterior sound to the eardrum.

The invention also provides an active in-the-ear hearing protector madesuitable to be sealingly inserted, one in each ear canal of a wearer,for protecting the wearer from loud sounds while enabling the wearer tohear sounds at non-harmful sound pressure levels, comprising:

(a) a casing having distal and proximal ends and an exterior surfacebetween said distal and proximal ends and having in its interior:

(i) microphone means for converting exterior sounds originatingexteriorly of the outer car to an electrical microphone signal,

(ii) electronic means for converting said electrical microphone signalto an electrical output signal, said electronic means includingelectronic compression means for controlling the amplitude of saidelectrical output signal to be proportional to the amplitude of saidelectrical microphone signal when the amplitude of said electricalmicrophone signal is below a pre-determined threshold value, and forautomatically limiting the amplitude of said electrical output signal towithin a pre-determined narrow dynamic range when the amplitude of saidelectrical microphone signal increases beyond said pre-determinedthreshold value so that the amplitude of said electrical output signalis not allowed to exceed a pre-determined maximum amplitude,

(iii) sound reproducer means for producing sound waves in response tosaid electrical output signal,

(iv) sound transmitting means sealingly attached to the acoustic outletof said sound reproducer means and to the wall of said distal end ofsaid casing for delivering sound waves produced by said sound reproducermeans to the exterior of said distal end of said casing, wherebytransmission of said exterior sounds through said easing and then saidsound transmitting means is minimised,

(v) battery means for supplying power for operation of said activehearing protector,

(b) an on/off switching means which is partly located exteriorly to andproximally of said casing, whereby the electrical power supplied by saidbattery can be switched on or off exteriorly to said easing at theproximal end of said easing,

(c) a user detachable ear piece comprising:

(i) a sound transmitting tube,

(ii) a polymeric retarded recovery foam member having distal andproximal ends, said foam member sealingly surrounding said soundtransmitting tube, whereby an acoustic seal is formed between said soundtransmitting robe and said foam member especially when said foam memberis mechanically compressed to miniraise transmission of said exteriorsounds to the eardrum, said foam member defining a substantiallycylindrical clement having a slow compression recovery rate, wherebysaid ear piece may be mechanically compressed and inserted distal endfirst into an ear canal of the wearer and will thereupon expand to forman elongated acoustic seal therein between the ear piece and asubstantial surface area of the ear canal to miniraise transmission ofsaid exterior sounds to the eardrum,

(iii) connector means attached to said proximal end of said polymericfoam member for detachably and sealingly attaching said detachable carpiece to the distal portion of said easing and for sealingly connectingsaid sound transmitting means attached to said sound reproducer means tosaid sound transmitting tube in said detachable ear piece, wherebysounds produced by said sound reproducer means are transmitted throughan acoustically sealed sound transmission path comprising said soundtransmitting means in said casing and said sound transmitting tube insaid car piece to the ear canal and eardrum so that the transmission ofsaid exterior sounds through said connector means to the eardrum isminimized.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is further described by way of example only with referenceto the accompanying drawings, in which:

FIG. 1 is a diagrammatic view of the hearing protector embodying thepresent invention.

FIG. 2 is a diagrammatic view illustrating the insertion of the hearingprotector of FIG. 1 embodying the invention into the car canal with thedetachable car piece of the protector in its mechanically compressedstate;

FIG. 3 is a diagrammatic view illustrating the hearing protector of FIG.1 after its detachable ear piece has expanded to engage thecircumference of a portion of the car canal of a wearer;

FIG. 4 illustrates a most general block diagram of the electronics of anonlinear active hearing protector;

FIG. 5 is a schematic section through an in-the-ear hearing protectordesigned in accordance with the present invention;

FIG. 6 is a graph presenting the difference in insertion loss measuredon a manikin between a conventional foam earplug and the hearingprotector of the present invention with its electrical power switchedoff;

FIG. 7 is a graph presenting the sound pressure levels produced by thesmall loudspeaker of the hearing protector of the present invention atthe artificial eardrum of a manikin for a number of values of soundpressure level at the artificial pinna.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The hearing protector 11 shown in FIG. 1 comprises a casing 12 and anattached ear piece 13. Thus, the casing 12 extends from a proximal endto a distal end at which it is connected to a proximal end of the earpiece 13. The ear piece is formed of an earplug member made of aretarded recovery foam which can be compressed, as shown in FIG. 2, topermit the ear piece to be inserted distal end first into the ear canalof a user, such that when the earplug member recovers by subsequentlyexpanding, it seals against the surface of the ear canal as shown inFIG. 3. In this condition, the casing 12 is outermost and the hearingprotector 11 is ready for use.

The casing 12 incorporates an active sound processor formed fromelectronic/electroacoustic components as shown in FIG. 4. These includea microphone 14, an electronic circuit 15, a small loudspeaker 16, abattery 17 and an on/off switch 18. Microphone 14 converts the incomingsound wave originating exteriorly of the outer ear to an electricalsignal which is processed by electronic circuit 15 which contain anonlinear compression amplifier and other signal conditioning circuits.For incoming sound waves at sound pressure levels which normally producelow to medium loudness in the absence of the protector, theamplification characteristic of the compression amplifier isapproximately linear, with a fixed gain. For incoming sound waves atsound pressure levels which normally produce loud hearing sensations,the amplification characteristic of the compression amplifier isnonlinear in that its gain is inversely related to the sound pressurelevel of the incoming sound wave.

The electrical output signal of electronic circuit 15 is delivered toloudspeaker 16 which converts the electrical signal back to sound insidethe hearing protector 11 in the ear canal. The electronic components ofthe electronic circuit 15 are chosen so that, for incoming sound waveswhich normally produce low to medium loudness, the sound pressure levelin the ear canal produced by loudspeaker 16 is approximately the same asthat produced by the incoming sound wave in the absence of theprotector. On the other hand, for incoming sound waves which normallyproduce loud hearing sensations, the sound pressure level in the earcanal produced by loudspeaker 16 is less than that produced by theincoming sound wave in the absence of the protector, and is not allowedto exceed a maximum level, to protect the wearer from harmful noiseexposure. Battery 17 provides the power source for hearing protector 11,which may be turned on or off by operating switch 18.

Referring now to FIG. 5, the casing 12 has an proximal section 12a, adistal section 12b and a face plate 19. Section 12a is formed with alongitudinal wall 45 which is somewhat cylindrical, the face plate 19 atits proximal end, and a transverse wall 41 at its distal end. Transversewall 41 has a hole, which is offset from the geometric centre of thewall.

Face plate 19 is secured to section 12a of casing 12 by a suitablesolvent 20. Sections 12a and 12b are moulded as one plastic piece.

Battery 17 is located inside the casing 12, in a battery holder 23closed by a removable battery cover 24. Holder 23 and cover 24 areintegral parts of face plate 19. Electronic circuit 15 of the hearingprotector is mounted in the easing 12 on face plate 19, and is connectedto switch 18, which is also mounted internally on face plate 19, butsuch that its actuating arm extends exteriorly through an opening 25 inface plate 19. The sound inlet 14a of a miniature heating aid microphone14 is tightly inserted into an orifice 22 through the longitudinal wall45 of section 12a and is further secured by applying solvent 26 on thewall of sound inlet 14a before the insertion. The body of microphone 14is secured on the interior surface of section 12a by using paste 27.

Cylindrical casing section 12b projects outwardly from transverse wall41 of casing section 12a, located opposite face plate 19. This definesan internal cavity which communicates with the interior of section 12athrough the hole in transverse wall 41 but which is substantially closedby an end wall 42 of section 12b. Casing section 12b has a circularorifice 21 through the end wall 42. A short piece of flexible plastictube 28 with an inner diameter mailer than the outside diameter of thesound outlet 16a of loudspeaker 16 and outside diameter greater than thediameter of circular orifice 21 is tightly inserted into orifice 21 soas to project a short distance into the cavity in distal section 12b,and is further secured by applying solvent 29 on the wall of robe 28before the insertion. A miniature hearing aid loudspeaker 16 is fittedinto a rubber boot 30 to reduce the effects of shock and vibration andthese are both positioned in the internal cavity in sections 12a and12b. The sound outlet 16a of loudspeaker 16 is tightly inserted into theend of flexible plastic robe 28 which is inside section 12b. A hearingaid acoustic damper 31 which modifies the frequency responsecharacteristics of hearing protector 11 to approximate the responsecharacteristics for normal hearing is inserted into flexible plastictube 28 from the outside of casing 12.

The external surface of section 12b of casing 12 is formed with a malescrew thread 32.

The ear piece 13 consists of an earplug member 33 made of polymericretarded recovery foam, which surrounds a flexible robe 34 with a flaredend 35. The ear piece also has a moulded plastic connector 36. Connector36 is substantially cup shaped, having a peripheral wall 43 andtransverse wall 44. Wall 44 has a centre hole 36a whose diameter is thesame as the outside diameter of flexible tube 34. A resilient washer 37is positioned in connector 36, within wall 43 on the inner side of wall44. Washer 37 has a centre hole whose diameter is the same as theoutside diameter of flexible tube 34. Wall 44 of connector 36 is bondedto earplug member 33 at one end of the earplug member remote from theflared end of the robe 34 by using solvent 38.

Wall 43 of connector 36 has an internal female screw thread 39, whichmates with male screw thread 32 of section 12b of casing 12 to join theconnector and ear pie together. The washer 37 is then compressed betweenwall 42 of section 12b of casing 12 and the wall 44 of connector 36, soas to provide a good acoustic seal between the easing 12 and car piece13. It will be realised that the resilient washer 37 in connector 36 maybe omitted, if a good enough acoustic seal between casing 12 and earpiece 13 can be established without the use of a washer.

Flexible robe 34 is snugly fitted into centre hole 36a of connector 36but does not protrude beyond centre hole 36a so that when ear piece 13is threaded onto casing 12, a continuous and uninterrupted passage isformed between the acoustic damper 31 and the opening of car piece 13 atflared end 35.

The construction of flared end 35 ensures that flexible tube 34 is notblocked by the ear plug member 33 and sound waves produced byloudspeaker 16 can reach the car canal and then the eardrum via the tube34.

The flexible tube 34 is not restricted to having the flared end 35 shownin FIG. 5. A flexible robe without a flared end may be used, such as onewhich extends for a short distance into the ear canal beyond the distalend of the ear plug member to avoid blockage of the robe by the ear plugmember.

Face plate 19 and its associated battery holder 23 and battery cover 24,electronic circuit 15, microphone 14, loudspeaker 16, rubber boot 30,acoustic damper 31 and on/off switch 18 may be formed of componentscustomarily used in standard in-the-ear hearing aids. Flexible plastictube 28 is most preferably a clear, flexible PVC tubing such as thoseused in standard hearing aid construction. Paste 27 is preferablysilicone rubber such as Silastic® Marine Sealant manufactured by DowComing. Solvents 20, 26, 29 and 38 are preferably cyanoacrylate instantadhesives such as Supa Glue manufactured by Selleys Chemical Company.Preferred materials from which casing 12 and connector 36 may be mouldedinclude Acrylonitrile-Butadiene-Styrenc (ABS) and High and Low DensityPolyethylene. Preferred materials for resilient washer 37 includenatural and synthetic rubber, cork, felt, and sponge rubber, see forexample C. E. Credo, "Vibration Isolation", Handbook of Noise Control,C. M. Harris Editor, 1957, McGraw-Hill Book Company, N.Y. Ear plugmember 33 is most suitably formed from a foam plastic that has retardedrecovery. Use of this material allows earplug member 33 to bemechanically compressed substantially but to re-expand into sealingengagement with the ear canal of a wearer within about 1-40 seconds. Theexterior surface of earplug member 33 preferably is substantiallycylindrical, tapering slightly inwardly towards its end which is innermost in the ear canal, to facilitate insertion into a wearers ear canal.Earplug member 33 may be formed by moulding, and its exterior surfacemay possess a somewhat smooth skin which is devoid of large cells thatcould pick up dirt and is therefore largely imperious to soil and liquidcontamination. One such foam earplug is described in U.S. Pat. No.4,774,938 (Leight). The earplug member of the heating protectoraccording to this invention is not restricted to being the form oftapered earplug member 33 shown in FIG. 5, but may for example becompletely

To receive flexible tube 34, earplug member 33 may be initiallycoaxially bored throughout its length with a central corer. Flexiblerobe 34 can be then inserted into the coaxial bore of earplug member 33.Alternatively, earplug member 33 may be moulded onto flexible tube 34during the moulding process for the manufacture of the earplug member.In either method, a fight fitting is formed between flexible robe 34 andearplug member 33.

Preferably, flexible tube 34 is made of soft plastic material such asSilastie® Medical Grade Tubing manufactured by Dow Coming; the internaldiameter of this tube preferably is large enough for good soundtransmission while its external diameter preferably is small enough toallow the fully compressed earplug member 33 to be easily insertableinto a human ear canal.

In operation, ear piece 13 is threaded onto casing 12 as described.Earplug member 33 is mechanically compressed and carefully inserted intothe ear canal by the wearer as illustrated in FIG. 2. Earplug member 33is allowed to expand and form a sealing engagement with the ear canal asillustrated in FIG. 3. The protector 11 is then switched on by means ofon/off switch 18.

The tight fitting between loudspeaker 16 and flexible plastic tube 28,and between flexible plastic tube 28 and orifice 21, provide acousticseals which reduce substantially the intensity of the incoming soundwave originating exteriorly of the outer ear, which may reach theacoustic damper end of flexible plastic tube 28 via the openings in faceplate 19, and may then reach the eardrum. As mentioned, when ear pie 13is threaded onto section 12b of casing 12, resilient washer 37 ismechanically compressed to provide an acoustic seal which reducessubstantially the intensity of the incoming sound wave originatingexteriorly of the outer ear, which may reach the washer end of flexibletube 34 via screw threads 32 and 39, and may then reach the eardrum. Thetight firing between flexible tube 34 and earplug member 33 provides anacoustic seal which reduces substantially the intensity of the incomingsound wave originating exteriorly of the outer ear, which may reach theear canal and then the eardrum via the coaxial bore of earplug member33. These acoustic seals assist in reducing the direct transmission ofincoming sound waves originating exteriorly of the outer ear to theeardrum to provide protection against harmfully loud noises by an mountapproximately equal to that provided by a passive foam earplug.

It will be realised that earplug member 33 is not restricted to beingformed from foam plastics, but may use other materials such as vinyls,cured silicones, and other elastomeric formations. When earplug member33 is formed from these alternative materials other than foam plastics,it will be realised that flexible tube 34 may not be needed and may bereplaced by an acoustic passage formed as a coaxial bore of the earplugmember during the moulding proems for the manufacture of the earplugmember.

It will also be realised that ear piece 13 is not restricted to beingattached to casing 12 by mating of screw threads on casing 12 andconnector 36, but may be attached to casing 12 by other means such as acoupler with an O-ring.

EXAMPLE

Eight prototype in-the-ear hearing protectors of the present inventionas illustrated in FIGS. 1 to 5 were constructed using standardin-the-ear hearing aid components. Each prototype used a KnowlesElectronics EM3056 microphone, a Knowles Electronics EH3054 receiver(small loudspeaker), a Genum LD511 compression amplifier, a KnowlesElectronics BF3039 acoustic damper, and a Microtronic SW521 on/offswitch. In addition, a 312 type zinc air battery, a face plate withdimensions suitable for receiving the 312 battery, and a rubber bootwith dimensions suitable for receiving the EH3054 receiver were used. Aclear PVC tube with an outside diameter of 2 mm and an internal diameterof 1.4 mm was used as flexible plastic tube 28 in FIG. 5. Silastic®Medical Grade Tubing 602-175 manufactured by Dow Corning with an outsidediameter of 1.65 mm and an internal diameter of 0.76 mm was used asflexible tube 34. The casing and connector were constructed on a latheusing clear acrylic material. The face plate 19 was 2 mm in thickness,with overall dimensions of 18 mm×12 mm. The wall of the casing was 1 mmin thickness and had overall outside dimensions of 18 mm×12 mm×5 mm forsection 12a; the outside diameter of the section 12b was 9 mm and itsoverall height is 3 mm. The cylindrical wall of the cylindricalconnector was 1.5 mm in thickness, and has an internal diameter of 9 mm.The bottom surface at the earplug member end of the connector had athickness of 0.5 mm. The overall height of the connector was 3.5 mm. Thescrew threads were formed using 9 mm tap and die with 0.75 mm pitch. Theresilient washer was formed by punching cylinders out of a 0.8 mm thicksheet of rubber. Supa Glue manufactured by Selleys and Silastie® MaxineSealant manufactured by Dow Corning were used. The earplug member wasformed using a Bilsom Form foam earplug manufactured by Bilsom. The foamearplug, approximately 23 mm in length, was initially coaxially boredthroughout its length with a central corer. The Silastic® flexible tubewas then inserted into the coaxial bore of the earplug member. TheSilastic® flexible tube extended approximately 0.6 mm beyond the distalend of the earplug member to avoid blocking of the sound transmissionpath by the foam material.

Testing was conducted on a Knowles Electronics Manikin for AcousticResearch with artificial pinnas, artificial ear canals and artificialeardrums. Test signals were one-third octave bandpass noise at centrefrequencies of 125 Hz to 8 kHz, and impulse noise generated by a toypistol with cap at a distance of 5 cm from the left artificial pinna.All sound pressure levels were measured A-weighted. For bandpass noisemeasurements, a measuring time constant corresponding to "RMS FAST" wasused; for impulse noise measurements, peak sound pressure levels wererecorded. The sound pressure level at the left artificial pinna isdesignated P_(pinna), and was monitored using a half-inch condensermicrophone.

Four sound pressure level measurements were made at the left artificialeardrum: (1) P_(open) for an unobstructed left artificial ear canal; (2)P_(foam) for a Bilsom Form foam earplug alone in left artificial earcanal; (3) P_(plugoff) for an active earplug of the present inventionwith its electrical power switched off in the left artificial ear canal;and (4) P_(plugon) for an active earplug of the present invention withits electrical power switched on in the left artificial ear canal. BothBilsom foam and active earplug of the present invention were initiallycompressed and inserted into the artificial ear canal, and were thenallowed to expand and form an acoustic seal with the canal.

The difference in insertion loss between the Bilsom foam alone and theactive earplug of the present invention with its electrical powerswitched off is shown in FIG. 6 as a function of the centre frequency ofthe one-third octave bandpass noise. All insertion loss measurementswere made with P_(pinna) fixed at a level of 100 dBA. Insertion loss forthe Bilsom foam alone is designated ILF, and is deemed as ILF(dBA)=P_(open) (dBA)-P_(foam) (dBA). Insertion loss for the activeearplug with its power switched off is designated ILP, and is defined asILP (dBA)=P_(open) (dBA)-P_(plugoff) (dBA). ILF was measured for eightBilsom foams; the eight foams correspond respectively to the eightprototype active earplug. After the ILF measurements (and the impulsenoise measurements to be described later) on the eight foams werecompleted, the detachable ear pieces of the corresponding eightprototypes were constructed by inserting a Silastic® tube into each foamafter the foam had been coaxially bored, and by securing the connectoron the proximal end of the foam using Supa Glue as described earlier.ILP measurements were then conducted.

In FIG. 6, each data point and the error bar associated with itrepresent the mean and standard deviation of the eight values of(ILP-ILF) for the eight prototypes, respectively. From FIG. 6, it can beseen that the difference in insertion loss measured on the manikinbetween the Bilsom Form foam earplug and the prototype earplug of thepresent invention with its electrical power switched off is small--thatis, the prototype earplug with its power switched off providesapproximately the same mount of attenuation and therefore protection asthe Bilsom Form foam earplug, indicating that the acoustic sealsimplemented in the prototype earplugs were effective.

FIG. 7 shows the variations of P_(plugon) for one of the prototypeearplugs versus the centre frequency of the one-third octave bandpassnoise for a number of levels of P_(pinna). The variations of P_(open)over frequency for P_(pinna) at 60 dBA and 90 dBA are also shown forcomparison. The electronic components of the prototypes were sealed sothat, for P_(pinna) below 80 dBA, P_(plugon) produced by the receiver inthe prototype is approximately the same as P_(open) ; on the other hand,for P_(pinna) above 80 dBA, P_(plugon) does not increase linearly withrespect to P_(pinna) and is prevented from increasing to levels beyondP_(open) recorded for P_(pinna) at 90 dBA. From FIG. 7, it can be seenthat the levels of P_(plugon) for a fixed P_(pinna) of 60 dBAapproximate those of P_(open) for the same level of P_(pinna) from 250Hz to 4 kHz, the major frequency band for speech and common everydaysounds. For levels of P_(pinna) between 60 and 80 dBA, P_(plugon)increases approximately linearly for all the frequencies tested.P_(plugon) is, however, strongly limited for levels of P_(pinna) above80 dBA, and does not exceed P_(open) recorded for P_(pinna) at the levelof 90 dBA. These characteristics shown in FIG. 7 were also observed forthe other seven prototypes. The experimental results therefore showedthat the active earplug of the present invention offers unattenuatedcommunication for speech and warning signals at low to medium soundpressure levels, and provides protection against harmfully loud noises.P_(open), P_(foam), P_(plugon), and P_(plugoff) were measured for two ofthe prototypes at the left artificial eardrum when a toy pistol with capwas fired 5 cm from the left artificial pinna. Note that, as mentionedearlier, P_(foam) was measured for each prototype before theconstruction of the detachable ear piece. The results of these peaksound pressure level measurements arc similar for the two prototypes andtheir results arc pooled and shown in TABLE I. Each entry in the Tablerepresents the mean of ten peak sound pressure measurements. From TABLEI, it can be seen that for a loud impulse noise such as a pistol shotthe levels of P_(foam), P_(plugon) and P_(plugoff) are approximately thesame. The prototype active earplug of the present invention is thereforeeffective in protecting the wearer from loud impulse noises.

                  TABLE 1                                                         ______________________________________                                        P.sub.open                                                                              P.sub.foam   P.sub.plugoff                                                                          P.sub.plugon                                  ______________________________________                                        166 dBA   105 dBA      103 dBA  106 dBA                                       ______________________________________                                    

The described form of the invention comprises:

(1) a small casing having on its wall an on/off switch and in itsinterior a microphone, an electronic circuit, a battery, a smallloudspeaker (also known as a receiver), and a flexible plastic tubetightly fitted at one end to the acoustic outlet of the loudspeaker andat the other end to an orifice on the wall of the casing, and

(2) a detachable ear piece comprising an earplug member made ofpolymeric retarded recovery foam material, a flexible sound transmissiontube sealingly secured inside the earplug member, and a connector whichis permanently attached to one end of the earplug member.

Preferably, the detachable ear piece is attached to the casing by matingof screw threads on the connector and the casing. An acoustically sealedsound mission path is formed from the outlet of the small loudspeaker inthe casing to the sound transmission tube in the ear piece by theinclusion of a resilient washer which is mechanically compressed betweenthe casing and the connector by the mating of the screw threads.

In operation, the detachable ear piece is connected to the casing bymeans of the connector on the ear piece. The earplug member made of foammaterial is mechanically compressed and carefully inserted into the earcanal. The earplug member is allowed to expand and form an acousticallysealing engagement with the ear canal. The active hearing protector isswitched on by means of the on/off switch. The microphone converts theincoming sound wave originating exteriorly of the outer ear to anelectrical signal which is processed by the electronic circuitcontaining a nonlinear compression amplifier and other signalconditioning circuits. The electrical output signal of the electroniccircuit is delivered to the small loudspeaker which converts theelectrical signal back to sound. The sound wave produced by theloudspeaker propagates through an acoustically sealed sound transmissionpath comprising successively the flexible plastic tube attached to theoutlet of the loudspeaker in the casing, the resilient washer, theconnector, and the sound transmission tube inside the earplug member ofthe detachable ear piece, to reach the ear canal and the eardrum. Theelectronic components of the electronic circuit are chosen so that, forincoming sound waves which normally produce low to medium loudness, thesound pressure level in the ear canal produced by the loudspeaker isapproximately the same as that produced by the incoming sound wave inthe absence of the protector. On the other hand, for incoming soundwaves which normally produce loud hearing sensations, the sound pressurelevel in the ear canal produced by the loudspeaker is less than thatproduced by the incoming sound wave in the absence of the protector, andis not allowed to exceed a maximum level to protect the wearer fromharmful noise exposure.

The described hearing protector reduces substantially the directtransmission of incoming sound waves originating exteriorly of the outerear to the eardrum by the acoustic seals provided by:

(1) the tight flags between the flexible plastic tube and the acousticoutlet of the small loudspeaker in the casing, and between the tube andthe wall of the casing,

(2) the novel design of the connector including the resilient washer onthe detachable car piece,

(3) the tight fitting between the flexible sound transmission robe andthe earplug member of the detachable car piece, and

(4) the tight fitting between the earplug member and the car canal.

The sound intensity at the eardrum produced by the incoming sound waveoriginating exteriorly of the outer car after propagating through theseacoustic seals is therefore very much attenuated. The hearing protectorof the present invention offers unattenuated communication for speechand warning signals at low to medium sound pressure levels against aquiet background, and provides protection against harmfully loud noisesby an amount approximately equal to that provided by a passive foamearplug.

The size of the novel heating protector of the present invention issmall and can be fined into the outer car of the wearer in the samemanner as an in-the-car hearing aid or nonlinear-active hearingprotectors in the form of in-the-car hearing aids. The electroacousticand electronic components of the hearing protector of the presentinvention arc those used in conventional in-the-car hearing aids.

The hearing protector described with reference to the drawings offers anumber of advantages over conventional nonlinear-active hearingprotectors in the form of in-the-car hearing aids using custom-madeearmoulds. The earplug member made of foam material of the detachablecar piece of the present invention guarantees that the hearing protectoris always snugly fitted into the car canal, and the presence of theconnector and the casing provides a convenient mechanical means forinserting and extracting the hearing protector from the outer ear. Thehearing protector of the present invention can be conveniently acquiredoff-the-shelf without specialised fitting by trained technicians asrequired by heating protectors using custom-made earmoulds. Clogging ofthe sound transmission tube in the detachable ear piece of the heatingprotector of the present invention by ear wax can be easily cleanedbecause the detachable ear piece can be easily detached from the casingso that both ends of the sound transmission robe are accessible to theuser, and the ear wax can be easily extracted from within the tube bythe use of a simple tool such as a fine metallic wire. The detachableear piece of the hearing protector of the present invention is easy tomanufacture and is inexpensive to the user; it can be readily replacedif its earplug member becomes soiled or dirty or if its soundtransmission tube is irreparably clogged by ear wax. The casingcontaining the electronic and electroacoustic components is transferablefrom user to user.

The hearing protector described with reference to the drawing is anonlinear active in-the-car hearing protector which reducessubstantially the direct Transmission of incoming sound wavesoriginating exteriorly of the outer ear to the eardrum by using acousticseals.

The hearing protector described with reference to the drawings providesa nonlinear active in-the-ear hearing protector which converts incomingsound waves originating exteriorly of the outer ear to an electricalsignal by means of a microphone, processes the electrical signals bymeans of an electronic circuit, converts the processed electricalsignals back to sound waves by means of a small loudspeaker, anddelivers the processed sound waves which are now at non-harmful soundpressure levels to the car canal and then the ear drum.

The hearing protector described with reference to the drawings providesa nonlinear active in-the-ear hearing protector with a detachable earpiece which provides a fight fitting between a earplug member on the earpiece and the ear canal, and is easy to insert into and extract from theouter ear.

The hearing protector described with reference to the drawing provides anonlinear active in-the-ear hearing protector which can be convenientlyacquired off-the-shelf without specialised fitting by trainedtechnicians.

The hearing protector described with reference to the drawing provides anonlinear active in-the-car hearing protector with a detachable carpiece which is easy to manufacture and is inexpensive to the user; thedetachable car piece can be easily cleaned by detaching it from thehearing protector, and can be readily replaced if it becomes soiled ordirty to maintain a high degree of hygiene, or replaced if its soundtransmission path is irreparably clogged by car wax.

The hearing protector described with reference to the drawings providesa nonlinear active in-the-ear hearing protector with a casing whichcontaining electronic and electroacoustic components, and istransferable from user to user.

Although hearing protector 11 has been described as a single unit foruse in protecting the hearing of one ear of a wearer, it should beapparent that two hearing protectors should normally fly be worn by thewearer, one in each ear, to protect both ears from damage caused byexposure to loud sounds.

While there are shown and described present preferred embodiments of theinvention it shall be clearly understood that such description is forillustrative purposes only and is not to be construed in a limitingsense since many modifications and alterations may be made withoutdeparting from the spirit and scope of the invention which includesevery novel feature and combination of features herein disclosed.

The claims defining the invention are as follows:
 1. An activein-the-car hearing protector made suitable to be sealingly inserted, onein each ear canal of a wearer, for protecting the wearer from loudsounds while enabling the wearer to hear sounds at non-harmful soundpressure levels, comprising:(a) a casing having distal and proximal endsand an exterior surface between said distal and proximal ends and havingin its interior:(i) microphone means for converting exterior soundsoriginating exteriorly of the outer eat to an electrical microphonesignal, (ii) electronic means for converting said electrical microphonesignal to an electrical output signal, said electronic means includingelectronic compression means for controlling the amplitude of saidelectrical output signal m be proportional to the amplitude of saidelectrical microphone signal when the amplitude of said electricalmicrophone signal is below a pre-determined threshold value, and forautomatically limiting the amplitude of said electrical output signal towithin a pre-determined narrow dynamic range when the amplitude of saidelectrical microphone signal increases beyond said pre-determinedthreshold value so that the amplitude of said electrical output signalis not allowed to exceed a pre-determined maximum amplitude, (iii) soundreproducer means for producing sound waves in response to saidelectrical output signal, (iv) sound transmitting means sealinglyattached to the acoustic outlet of said sound reproducer means and tothe wall of said distal end of said casing for delivering sound wavesproduced by said sound reproducer means to the exterior of said distalend of said casing, whereby Transmission of said exterior sounds throughsaid casing and then said sound transmitting means is minimized, (v)battery means for supplying power for operation of said active hearingprotector, (b) an on/off switching means which is partly locatedexteriorly to and proximally of said easing, whereby the electricalpower supplied by said battery can be switched on or off exteriorly tosaid casing at the proximal end of said casing, (c) a user detachableear piece comprising:(i) a sound transmitting tube, (ii) a polymericretarded recovery foam member having distal and proximal ends, said foammember sealingly surrounding said sound transmitting tube, whereby anacoustic seal is formed between said sound transmitting tube and saidfoam member especially when said foam member is mechanically compressedto miniraise transmission of said exterior sounds to the eardrum, saidfoam member defining a substantially cylindrical element having a slowcompression recovery rate, whereby said ear piece may be mechanicallycompressed and inserted distal end first into an ear canal of the wearerand will thereupon expand to form an elongated acoustic seal thereinbetween the ear piece and a substantial surface area of the ear canal tominimise transmission of said exterior sounds to the eardrum, (iii)connector means attached to said proximal end of said polymeric foammember for detachably and scalingly attaching said detachable car pieceto the distal portion of said casing and for sealingly connecting saidsound transmitting means attached to said sound reproducer means to saidsound transmitting tube in said detachable car piece, whereby soundsproduced by said sound reproducer means are transmitted through anacoustically sealed sound transmission path comprising said soundtransmitting means in said casing and said sound transmitting tube insaid ear piece to the ear canal and eardrum so that the transmission ofsaid exterior sounds through said connector means to the eardrum isminimised.
 2. An active in-the-car hearing protector as claimed in claim1, wherein said sound transmitting tube in said foam member is aflexible tube with a diameter substantially smaller than the diameter ofthe car canal, whereby case of insertion of said car piece into carcanal can be achieved.
 3. An active in-the-car hearing protector asclaimed in claim 1, wherein said sound transmitting tube extends for ashort distance beyond said distal end of said foam member into the earcanal, whereby said sound transmitting tube is not blocked by the foammaterial at said distal end of said foam member to ensure that soundsproduced by said sound reproducing means can reach the ear canal withoutany inadvertent obstruction.
 4. An active in-the-car heating protectoras claimed in claim 1 wherein said sound transmitting tube has anoutwardly flared end at said distal of said foam member, whereby saidsound transmitting tube is not blocked by the foam material at saiddistal end of said foam member to ensure that sounds produced by saidsound reproducing means can reach the car canal without any inadvertentobstruction.
 5. An active in-the-ear hearing protector as claimed inclaim 1, wherein said casing is made of plastic.
 6. An active in-the-earhearing protector as claimed in claim 1, wherein said distal portion ofsaid casing is cylindrical with a flat bottom surface at said distalend.
 7. An active in-the-car hearing protector as claimed claim 1wherein said sound transmitting means in said casing is a flexibleplastic tube sealingly inserted into a centre hole in said bottomsurface but does not extend beyond said distal end of said casing.
 8. Anactive in-the-ear hearing protector as claimed in claim 1 wherein saidexterior surface of said distal portion of said casing is formed with amale screw thread.
 9. An active in-the-car hearing protector as claimedof claim 1, wherein said connector means comprises a hollow cylindricalconnector which is closed at the distal end of said connector means by afiat end surface, a centre hole in said end surface concentric with saidcentre hole in said bottom surface at said distal end of said casing,and a resilient washer with a centre hole concentric with said centrehole in said end surface.
 10. An active in-the-car hearing protector asclaimed in claim 9, wherein said hollow cylindrical connector is made ofplastic.
 11. An active in-the-ear hearing protector as claimed in claim9, wherein said resilient washer is contiguous with said end surface.12. An active in-the-ear hearing protector as claimed in claim 9 whereinthe interior surface of said hollow cylindrical connector is formed witha female screw thread for mating with said male screw thread on saiddistal portion of said casing.
 13. An active in-the-ear hearingprotector as claimed in claim 9 wherein said sound transmitting tube insaid detachable eat piece is inserted through said centre hole in saidend surface of said hollow cylindrical connector but does not extendbeyond said end surface, whereby when said detachable ear piece isattached to said casing by mating of said male and female screw threadsan acoustically sealed sound transmission path is formed from said soundreproducing means to the ear canal and eardrum by mechanicallycompressing the resilient washer between said end surface of said hollowcylindrical connector and said bottom surface of said casing.
 14. Anactive in-the-car hearing protector as claimed in claims 9 wherein saidresilient washer is made of rubber.
 15. An active in-the-car hearingprotector as claimed in claim 9 wherein said resilient washer is made ofcork.
 16. An active in-the-car hearing protector as claimed in claim 9wherein said resilient washer is made of felt.
 17. An active in-the-carhearing protector as claimed in claim 1 wherein said connector meanscomprises a hollow cylindrical connector which is closed at the distalend of said connector means by a fiat end surface, and a centre hole insaid end surface concentric with said centre hole in said bottom surfaceat said distal end of said casing.
 18. An active in-the-car hearingprotector as claimed in claim 17, wherein said hollow cylindricalconnector is made of plastic.
 19. An active in-the-car hearing protectoras claimed in claim 17 wherein the interior surface of said hollowcylindrical connector is form female screw thread for mating with saidmale screw thread on said distal portion of said casing.
 20. An activein-the-ear hearing protector as claimed in claim 17 wherein said soundtransmitting robe in said detachable ear piece is inserted through saidcentre hole in said end surface of said hollow cylindrical connector butdoes not extend beyond said end surface, whereby when said detachableear piece is attached to said casing by mating of said male and femalescrew threads an acoustically sealed sound mission path is formed fromsaid sound reproducing means to the ear canal and eardrum bycontiguously attaching said end surface of said hollow cylindricalconnector to said bottom surface of said casing.
 21. A hearing protectorcomprising a sound processor for receiving at an input thereof exteriorsound and generating therefrom at an output thereof correspondingprocessed sound limited to intensity not exceeding a predetermined soundpressure level, and an earpiece detachably mounted to said soundprocessor and having sound channeling means in the form of a tube forpassage of said processed sound away from said sound processor, saidhearing protector being insertable into an ear canal so as to positionsaid sound processor for receiving at said input said exterior sound,being sound originating exteriorally of said ear canal, and fordirecting said processed sound, as generated from said exterior sound bysaid sound processor, to an eardrum via said sound channeling means ofsaid earpiece, said earpiece having first acoustic sealing means formedof a retarded recovery resilient material which is compressible toenable the earpiece to be inserted into the ear canal, but which thenrecovers to form a tight first acoustic seal between the earpiece andthe ear canal, such as to preclude sound from passing from the outer earthrough the earpiece to the eardrum via the ear canal otherwise thanthrough the sound channeling means, second acoustic sealing means beingprovided effective to provide a second acoustic seal between the outputof said sound processor and an adjacent input end of the soundchanneling means, to preclude sound other than said processed sound fromentering the sound channeling means, said retarded recovery resilientmaterial sealingly surrounding said tube to form a third acoustic sealbetween the ear plug member and the exterior of the sound channelingmeans, said acoustic seals being effective to preclude directtransmission of said exterior sound to the eardrum.
 22. A hearingprotector as claimed in claim 21 having a casing which incorporates saidsound processor and which is attached to said earpiece by a connectormeans, said casing having an acoustic inlet positioned to receive saidexterior sound when said casing is attached to said earpiece and whensaid hearing protector is inserted into the outer ear.
 23. A hearingprotector as claimed in claim 22 wherein said connector means holds saidsecond acoustic sealing means between the casing and the earpiece so asto provide said second acoustic seal.
 24. A hearing protector as claimedin claim 23 wherein said second acoustic sealing member comprises aresilient washer which is axially compressed by said connector means.25. A hearing protector as claimed in claim 21 wherein said soundprocessor comprises a microphone to receive said exterior sound, anelectronic circuit and a loudspeaker, said electronic circuit beingcoupled to receive microphone electrical signal generated by saidmicrophone pursuant to incidence of said exterior sound thereon and topass an output electrical signal to said loudspeaker for generation ofsaid processed sound for passage into said sound channeling means, saidelectronic circuit being arranged to limit said output electrical signalas passed therefrom to said loudspeaker such as not to exceed a valuewhich causes intensity of said processed sound to exceed saidpredetermined sound pressure level.
 26. A hearing protector comprising asound processor for receiving at an input thereof exterior sound andgenerating therefrom at an output thereof corresponding processed soundlimited to intensities not exceeding a predetermined sound pressurelevel, and an ear piece detachably mounted to said sound processor andhaving sound channeling means for passage of said processed sound awayfrom said sound processor, said hearing protector being insertable intothe ear canal so as to position said sound processor for receiving atsaid input said exterior sound, being sound originating exteriorly ofthe outer ear, and for directing said processed sound, as generated fromsaid exterior sound by said sound processor, to the ear drum via saidsound channeling means of said ear piece, said ear piece having firstacoustic sealing means formed of a retarded recovery resilient materialwhich is compressible to enable the ear piece to be inserted into theear canal, but which then recovers to form a tight first acoustic sealbetween the ear piece and the ear canal, such as to preclude sound frompassing from the outer ear through the ear piece to the ear drum via theear canal otherwise than through the sound channeling means, secondacoustic sealing means being provided effective to provide a secondacoustic seal between the output of said sound processor and an adjacentinput end of the sound channeling means, to preclude sound other thansaid processed sound from entering the sound channeling means, saidacoustic seals being effective to preclude direct transmission of saidexterior sound to the ear drum, said sound processor being incorporatedinto a casing which is attached to said ear piece by a connector means,said casing having an acoustic inlet positioned to receive said exteriorsound when said casing is attached to said ear piece and when saidhearing protector is inserted into the outer ear, said connector meansholding said second acoustic sealing means between the casing and theear piece so as to provide said second acoustic seal.
 27. A hearingprotector comprising a sound processor for receiving at an input thereofexterior sound and generating therefrom at an output thereofcorresponding processed sound limited to intensities not exceeding apredetermined sound pressure level, and an ear piece detachably mountedto said sound processor and having sound channeling means for passage ofsaid processed sound away from said sound processor, said hearingprotector being insertable into the ear canal so as to position saidsound processor for receiving at said input said exterior sound, beingsound originating exteriorly of the outer ear, and for directing saidprocessed sound, as generated from said exterior sound by said soundprocessor, to the ear drum via said sound channeling means of said earpiece, said ear piece having first acoustic sealing means formed of aretarded recovery resilient material which is compressible to enable theear piece to be inserted into the ear canal, but which then recovers toform a tight first acoustic seal between the ear piece and the earcanal, such as to preclude sound from passing from the outer ear throughthe ear piece to the ear drum via the ear canal otherwise than throughthe sound channeling means, second acoustic sealing means being providedeffective to provide a second acoustic seal between the output of saidsound processor and an adjacent input end of the sound channeling means,to preclude sound other than said processed sound from entering thesound channeling means, said acoustic seals being effective to precludedirect transmission of said exterior sound to the ear drum, said soundprocessor being incorporated into a casing which is attached to said earpiece by a connector means, said casing having an acoustic inletpositioned to receive said exterior sound when said casing is attachedto said ear piece and when said hearing protector is inserted into theouter ear, said connector means holding holds said second acousticsealing means between the casing and the ear piece so as to provide saidsecond acoustic seal; wherein said sound processor comprises amicrophone to receive said exterior sound, an electronic circuit and aloudspeaker, said electronic circuit being coupled to receive microphoneelectrical signal generated by said microphone pursuant to incidence ofsaid exterior sound thereon and to pass an output electrical signal tosaid loudspeaker for generation of said processed sound for passage intosaid sound channeling means, said electronic circuit being arranged tolimit said output electrical signal as passed therefrom to saidloudspeaker such as not to exceed a value which causes intensity of saidprocessed sound to exceed said predetermined sound pressure level.
 28. Ahearing protector comprising a sound processor for receiving at an inputthereof exterior sound and generating therefrom at an output thereofcorresponding processed sound limited to intensities not exceeding apredetermined sound pressure level, and an ear piece detachably mountedto said sound processor and having sound channeling means for passage ofsaid processed sound away from said sound processor, said hearingprotector being insertable into the ear canal so as to position saidsound processor for receiving at said input said exterior sound, beingsound originating exteriorly of the outer ear, and for directing saidprocessed sound, as generated from said exterior sound by said soundprocessor, to the ear drum via said sound channeling means of said earpiece, said ear piece having first acoustic sealing means formed of aretarded recovery resilient material which is compressible to enable theear piece to be inserted into the ear canal, but which then recovers toform a tight first acoustic seal between the ear piece and the earcanal, such as to preclude sound from passing from the outer ear throughthe ear piece to the ear drum via the ear canal otherwise than throughthe sound channeling means, second acoustic sealing means being providedeffective to provide a second acoustic seal between the output of saidsound processor and an adjacent input end of the sound channeling means,to preclude sound other than said processed sound from entering thesound channeling means, said sound processor being incorporated into acasing which is attached to said ear piece by a connector means, saidcasing having an acoustic inlet positioned to receive said exteriorsound when said casing is attached to said ear piece and when saidhearing protector is inserted into the outer ear, said connector meansholding holds said second acoustic sealing means between the casing andthe ear piece so as to provide said second acoustic seal; said ear piececomprising an inner tube defining said sound channeling means and anearplug member, said earplug member sealingly surrounding said innertube to form a third acoustic seal between the earplug member and theexterior of the sound channeling means, said acoustic seals beingeffective to preclude direct transmission of said exterior sound to theear drum.
 29. A hearing protector comprising a sound processor forreceiving at an input thereof exterior sound and generating therefrom atan output thereof corresponding processed sound limited to intensitiesnot exceeding a predetermined sound pressure level, and an ear piecedetachably mounted to said sound processor and having sound channelingmeans for passage of said processed sound away from said soundprocessor, said hearing protector being insertable into the ear canal soas to position said sound processor for receiving at said input saidexterior sound, being sound originating exteriorly of the outer ear, andfor directing said processed sound, as generated from said exteriorsound by said sound processor, to the ear drum via said sound channelingmeans of said ear piece, said ear piece having first acoustic sealingmeans formed of an ear plug member formed of retarded recovery resilientmaterial which is compressible to enable the ear piece to be insertedinto the ear canal, but which then recovers to form a tight firstacoustic seal between the ear piece and the ear canal, such as topreclude sound from passing from the outer ear through the ear piece tothe ear drum via the ear canal otherwise than through the soundchanneling means, second acoustic sealing means being provided effectiveto provide a second acoustic seal between the output of said soundprocessor and an adjacent input end of the sound channeling means, topreclude sound other than said processed sound from entering the soundchanneling means, said sound processor being incorporated into a casingwhich is attached to said ear piece by a connector means, said casinghaving an acoustic inlet positioned to receive said exterior sound whensaid casing is attached to said ear piece and when said hearingprotector is inserted into the outer ear, said connector means holdingholds said second acoustic sealing means between the casing and the earpiece so as to provide said second acoustic seal; said ear piececomprising an inner tube defining said sound channeling means and anearplug member formed by said first acoustic means, said earplug membersealingly surrounding said inner tube to form a third acoustic sealbetween the earplug member and the exterior of the sound channelingmeans, said acoustic seals being effective to preclude directtransmission of said exterior sound to the ear drum; said soundprocessor comprising a microphone to receive said exterior sound, anelectronic circuit and a loudspeaker, said electronic circuit beingcoupled to receive microphone electrical signal generated by saidmicrophone pursuant to incidence of said exterior sound thereon and topass an output electrical signal to said loudspeaker for generation ofsaid processed sound for passage into said sound channeling means, saidelectronic circuit being arranged to limit said output electrical signalas passed therefrom to said loudspeaker such as not to exceed a valuewhich causes intensity of said processed sound to exceed saidpredetermined sound pressure level.